Liquid crystal display devices have come to be applied to, for example, watches, calculators, a variety of household electrical appliances, measuring equipment, panels used in automobiles, word processors, electronic notebooks, printers, computers, and television sets. Representative examples of types of liquid crystal display devices include a TN (twisted nematic) type, an STN (super twisted nematic) type, a DS (dynamic scattering) type, a GH (guest⋅host) type, an IPS (in-plane switching) type, an OCB (optically compensated birefringence) type, an ECB (electrically controlled birefringence) type, a VA (vertical alignment) type, a CSH (color super homeotropic) type, and an FLC (ferroelectric liquid crystal) type. A typical drive system is static driving; however, multiplex driving has become common, and a passive matrix and, in recent years, an active matrix (AM) in which a TFT (thin film transistor), a TFD (thin film diode), or another device is used for driving have become mainstream.
In general, liquid crystal display devices have a view angle dependency attributed to the birefringence properties of the liquid crystal molecules. In order to address the view angle dependency, an optical film (also referred to as optical compensation film) having different birefringence properties from liquid crystal molecules is used. In a liquid crystal display device in which rod-like liquid crystal molecules having a positive dielectric anisotropy are used, for example, a liquid crystal cell provided with just a polarizing plate has a problem with viewing angle characteristics, in which light leaking from the liquid crystal cell is observed when it is viewed in an oblique direction.
Techniques for addressing such a problem with viewing angle characteristics have been employed; for instance, one biaxial retardation layer is placed between a liquid crystal cell and each of upper and lower polarizing plates, both one uniaxial retardation layer and one completely axial retardation layer are placed on each of the upper and lower sides of a liquid cell, or a uniaxial retardation layer and a completely biaxial retardation layer are disposed on one side of a liquid crystal cell.
A liquid crystal display device of which a retardation layer is disposed outside the liquid crystal cell (out-cell type) has been common; however, in order to enhance productivity by reducing the thickness and weight of a liquid crystal display device and by eliminating an attachment step, a liquid crystal display device of which a retardation layer is disposed inside the liquid crystal cell (in-cell type) has recently come to be developed. Representative known examples of such a technique are, for instance, as follows: disposing a positive A plate inside a liquid crystal cell (see PTL 1), disposing a positive C plate inside a liquid crystal cell (PTL 2), and providing retardation layers of a positive A plate and positive C plate (see PTL 3).
Since the electric properties of a display device are greatly affected by impurities remaining in a liquid crystal material used in a liquid crystal layer, the impurity content has been highly controlled. Furthermore, it is known that an alignment film is directly in contact with a liquid crystal layer and that the electric properties of the liquid crystal layer are affected by the movement of impurities remaining in the alignment film to the liquid crystal layer; hence, the characteristics of the liquid crystal display device that are attributed to impurities remaining in a material used for forming the alignment film has been studied.
In an in-cell liquid crystal display device of which a retardation layer is present inside the cell, a transparent electrode layer and an alignment film are between the liquid crystal layer and the retardation layer, and the direct effect of the retardation layer on the liquid crystal layer has been believed to be greatly smaller than the direct effect of a material used for forming the alignment film. The thickness of the alignment film is, however, generally not more than 0.1 μm, and the thickness of the transparent electrode layer is substantially similar. The liquid crystal layer and the retardation layer are not necessarily completely separated from each other; thus, it is speculated that impurities remaining in the retardation layer in an in-cell type have an effect on the liquid crystal layer as in a material used for forming the alignment film. The retardation layer containing impurities that have passed through the alignment film and the transparent electrode may cause a reduction in the voltage holding ratio (VHR) of the liquid crystal layer and defective display such as generation of white spots due to increased ion density (ID), uneven alignment, and image-sticking. The effects of impurities remaining in the retardation layer on the liquid crystal layer, however, have not been studied.